This invention relates to checking the operation of thermal printers and in particular to the checking of the operation of thermal printers when used in franking machines.
Franking machines print a franking on mails items in which the franking includes the value of postage and the date of franking and may include further data such as an identifier for the franking machine and coded data for checking the validity of the franking. It has been common in franking machines to use printing devices having print wheels which can be selectively positioned to print the required variable data and a print element to print the unchanging data. The print wheels are mechanically coupled to means for setting them to the desired printing position and in addition means are usually provided to sense the position of the wheels, prior to effecting the printing operation, so that there is no doubt that when the printing operation takes place the correct data will be printed. The construction of such printers is such that, provided an inking roller is present, a printing impression is assured upon operation of the device. Such printing devices require the manufacture and assembly of a large number of mechanical components and in addition, with the use of electronic data processing and display devices in franking machines, it is necessary to provide electro-mechanical interfaces between the electronic circuits and the mechanical components for setting the printing devices. Accordingly it is desirable to use printing devices which do not require movement of mechanical components to select the data to be printed. A convenient type of printing device to use in place of the mechanical printing devices is a thermally operated printing device using a thermal transfer process. In such printing devices, a print head has a plurality of selectively heatable print elements which bear against the rear face of a transfer ribbon carrying a layer of transferable ink on its front face. The ink is carried in a material which is melted in the region of a heated element of the print head whereby the melted material is transfered to the surface of an envelope held in contact with the front face of the ribbon. The elements of the print head are usually disposed in a line and by repeatedly selecting elements of the print head as the ribbon and envelope pass the print head a desired pattern is printed on the envelope. The selection of the elements of the print head is controlled for example by a microprocessor to print a desired pattern containing the required franking data. Although such printing devices have high reliability , there is no assurance ,as is the case with previously used mechanical printing devices, that all the selected printing elements have operated correctly and hence it is possible for a franking to be effected which is defective due to failure of one or more of the printing elements to heat to a temperature sufficient to cause transfer of ink from the ribbon to the envelope. A defective franking may be such as to be unacceptable to the postal authority while the accounting apparatus of the franking machine will have debited the cost of the franking to the user of the machine.